The present invention relates to a fuel injection system equipped with electronic fuel control means and more particularly to a fuel injection system having lower and higher opening region throttle sensing systems such that in accordance with the electric signals generated from the two sensing systems and changeover between the sensing systems is automatically and smoothly effected to control the quantity of fuel injected from fuel injection valves.
Techniques of employing electronic circuitry to control the quantity of fuel injected by a fuel injection system are well known in the art as disclosed for example in JP-A-57-56632. This patent application discloses a fuel control method featuring that the quantity of fuel supplied to the engine is controlled in accordance with the amount of intake air flow detected from the rate of air flow detected by a hot-wire flow meter when the throttle opening detected by a throttle sensor is in the range from the idling opening to a given value, and the fuel quantity is controlled in accordance with a predetermined throttle opening corresponding to the output value of an engine speed sensor for detecting the speed of the engine and the amount of intake air flow corresponding to the engine speed.
An electronically-controlled fuel injection system to which the present invention is applicable will now be described with reference to FIG. 1.
The flow rate of air drawn from an air cleaner 1 is varied by a throttle valve 4 disposed in a throttle control section 2.
The throttle valve 4 is linked to an accelerator pedal 3 and operated by the driver.
The air (intake air) passed through the throttle valve 4 is supplied to a combustion chamber 9 of an engine 8 through a surge tank 5, an intake manifold 6 and an intake valve 7. The mixture burned in the combustion chamber 9 is discharged to the atmosphere through an exhaust valve 10 and an exhaust manifold 11. While a fuel injection valve 14 is fitted into the intake manifold 6 for each of the combustion chamber 9, it is possible to provide only a single fuel injection valve upstream of the throttle valve 4.
As shown in detail in FIG. 2, an electronic control unit 15 includes a microprocessor which functions as a computer, a read-only memory (ROM), a random-access memory (RAM), input and output devices (I/O ports), etc., and the control unit 15 receives input signals from a throttle sensor 16 for detecting the rotational angle of the throttle valve 4, a water temperature sensor 18 fitted into a water jacket 17, an intake air temperature sensor 20 for detecting the intake air temperature, a rotational angle sensor 32 for detecting the rotational angle of a distributor 12 coupled to the crankshaft to detect the rotational speed of the crankshaft coupled to a piston 21 through a connecting rod 22, an ignition switch 24, a starter switch 25, etc. The rotational angle sensor 23 includes a position detector for generating a pulse for every two revolutions of the crankshaft and an angle detector for generating a pulse for every given crank angle, e.g., 1.degree.. The fuel is forced to each fuel injection valve 14 by a fuel pump 31 from a fuel tank 30 through a fuel passage 29. In accordance with the various input signals, the electronic control unit 15 computes a fuel injection quantity and a fuel injection timing to apply a fuel injection pulse to the fuel injection valve 14 and also computes an ignition timing to supply a current to an ignition coil 32. The secondary current of the ignition coil 32 is supplied to a distributor 33 which in turn distributes it to the respective spark plugs.
FIG. 2 is a block diagram showing the construction of the electronic control unit 15 and the outputs of the water temperature sensor 18, the intake air temperature sensor 20 and the throttle sensor 16 are sent to an A/D converter 34 which in turn convert them to digital signals. An engine speed detecting circuit 35 counts the number of pulses applied within a given time from the angle detector of the rotational angle sensor 23 to generate a value proportional to the engine speed. Numeral 36 designates a clock generator for controlling a digital operation. The outputs of the ignition switch 24 and the starter switch 25 are temporarily stored in a latch circuit 37. A microprocessor 40 is connected to an ROM 42, an RAM 43 as well as the A/D converter 34, the engine speed detecting circuit 35 and the latch circuit 37 through a bus line 41 to compute a fuel injection quantity in accordance with a predetermined program. The value corresponding to this fuel injection quantity is stored in a fuel injection control circuit 44 so that when the stored value coincides with the number of the clock pulses applied, an output pulse is generated and this output pulse is supplied to the fuel injection valve 14 through a driver circuit 45 for driving the fuel injection valves. The flow rate of air passed through the intake system is obtained by calculation from the throttle opening obtained from the output of the throttle sensor 16 and the engine speed obtained from the rotational angle sensor 23. While the fuel injection quantity is computed in accordance with the previously mentioned air flow rate, when the output of the throttle sensor 16 is taken into the computer, it is converted from the analog value to a digital value and therefore the fuel injection quantity is handled as a discrete data in terms of minimum bits. In order to make uniform the resolution of inputted data for all the air flow rate, the throttle sensor 16 includes a lower-opening throttle sensing system and a higher-opening throttle sensing system as shown in FIG. 3. In the throttle sensor of FIG. 3, the lower opening system includes a resistor 52 and conductors 56 and 57 which are arranged on the base as shown in the Figure and the resistor 52 and the conductor 56 are electrically connected by a brush 54 disposed on a lever 62 which is mounted on a throttle valve shaft 63. At this time, if a constant voltage is applied across the terminals 57 and 59 of the resistor 52, as shown at a in FIG. 4, a lower-opening throttle sensor output voltage is applied across the terminals 58 and 59 in accordance with the rotational angle of the throttle valve shaft 63. On the other hand, the higher opening system includes similarly a resistor 51, a brush 53, a conductor 55, the conductor 57, etc., and a higher-opening throttle sensor output is generated across terminals 60 and 61 as shown in b in FIG. 4. As will been seen from FIG. 4, the slope of the straight line for the lower-opening sensing system is greater than that of the higher-opening sensing system, that is, the former is higher in throttle opening resolution than the latter.
Then, due to a positional shift caused between the throttle sensing system by a manufacturing error, the variations in resistance value among the resistors due to manufacturing errors or the like, in FIG. 4 any deviation in the throttle opening axis direction, variations of the ratio between the slopes of the straight line a (the lower opening line) and the straight line b (the higher opening line) or the like cannot be avoided from the hardware point of view. Thus, there is a disadvantage that if the lower-opening a track and the higher-opening b track are separately inputted and processed in the computer, upon the changeover between the lower-opening track and the higher-opening track the throttle sensor output is caused to vary stepwise or the slopes of the tracks (FIG. 4) are varied, thereby causing a rapid increase or deviation of the air-fuel ratio during the changeover between the lower-opening track and the higher-opening track with the resulting deterioration of the driving performance and the exhaust emission.